docs: def _sigma_pi_loE(self, Tp):
          """
          inclusive cross section for Tth < Tp < 2 GeV
          Fit from experimental data
          """
          m_p = self._m_p
          m_pi = self._m_pi
          Mres = 1.1883  # GeV
          Gres = 0.2264  # GeV
          s = 2 * m_p * (Tp + 2 * m_p)  # center of mass energy
          gamma = np.sqrt(Mres ** 2 * (Mres ** 2 + Gres ** 2))
          K = np.sqrt(8) * Mres * Gres * gamma
          K /= np.pi * np.sqrt(Mres ** 2 + gamma)
  
          fBW = m_p * K
          fBW /= (
              (np.sqrt(s) - m_p) ** 2 - Mres ** 2
          ) ** 2 + Mres ** 2 * Gres ** 2
  
          mu = np.sqrt(
              (s - m_pi ** 2 - 4 * m_p ** 2) ** 2 - 16 * m_pi ** 2 * m_p ** 2
          )
          mu /= 2 * m_pi * np.sqrt(s)
  
          sigma0 = 7.66e-3  # mb
  
          sigma1pi = sigma0 * mu ** 1.95 * (1 + mu + mu ** 5) * fBW ** 1.86
  
          # two pion production
          sigma2pi = 5.7  # mb
          sigma2pi /= 1 + np.exp(-9.3 * (Tp - 1.4))
  
          E2pith = 0.56  # GeV
          sigma2pi[np.where(Tp < E2pith)] = 0.0
  
          return (sigma1pi + sigma2pi) * 1e-27